This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP third generation partnership project        CB codebook        CDM code division multiplexing        DL downlink (eNB towards UE)        eNB E-UTRAN Node B (evolved Node B)        EPC evolved packet core        E-UTRAN evolved UTRAN (LTE)        FDD frequency division duplex        HARQ hybrid automatic repeat request        ITU International Telecommunication Union        LOS line of sight        LTE long term evolution of UTRAN (E-UTRAN)        MAC medium access control (layer 2, L2)        MIMO multiple input multiple output        MM/MME mobility management/mobility management entity        Node B base station        O&M operations and maintenance        OFDMA orthogonal frequency division multiple access        PDCP packet data convergence protocol        PHY physical (layer 1, L1)        RF radio frequency        RLC radio link control        RRC radio resource control        RRM radio resource management        SC-FDMA single carrier, frequency division multiple access        S-GW serving gateway        TDD time division duplex        Tx transmitter        UCA uniform circular array        UE user equipment, such as a mobile station or mobile terminal        UL uplink (UE towards eNB)        ULA uniform linear array        UMi urban microcell        UTRAN universal terrestrial radio access network        XP cross-polarized        
A communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA) has been specified within 3GPP. The DL access technique is OFDMA, and the UL access technique is SC-FDMA.
One specification of interest is 3GPP TS 36.300, V10.4.0 (2011-06), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 10)”, incorporated by reference herein in its entirety.
FIG. 1 reproduces FIG. 4-1 of 3GPP TS 36.300, and shows the overall architecture of the E-UTRAN system. The E-UTRAN system includes eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE (not shown). The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1 MME interface and to a Serving Gateway (SGW) by means of a S1 interface. The S1 interface supports a many-to-many relationship between MMEs/S-GW and eNBs.
The eNB hosts the following functions:                functions for RRM: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both UL and DL (scheduling);        IP header compression and encryption of the user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards the Serving Gateway;        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or O&M); and        a measurement and measurement reporting configuration for mobility and scheduling.        
In order to meet the growing data capacity needs of cellular communications, heterogeneous networks, which will contain a mixture of macro, pico and femto cells, will be deployed. Since the deployment conditions are different for these various cell types, the needs of the antenna array employed in each type of cell can vary as well. For example a macro cell will cover a much larger area than the pico or femto cells and hence will likely employ uniform linear arrays (ULAs) with a limited beamwidth in elevation since the macro cell will tend to be situated much higher than the pico or femto cells. In addition, the macro cell will likely be sectorized (e.g., three sectors) which makes the use of ULA a good match. Compared to the macro cell, the pico cell (it should be noted that the femto cell, which in most deployments will be inside a residence, has very similar requirements to the pico cell) will likely be closer to street-level (e.g., deployed on lamp posts), will need to serve much fewer UEs, will serve UEs with a much wider range of elevation angles and will need to be much lower cost than the macro cell. Hence for the pico cell a single sector deployment makes the most sense and hence an array type should be chosen with omni-directional antennas in mind. One example of a good array choice for a pico cell is the uniform circular array (UCA) which has as an added bonus over the ULA with the ability to steer a beam in elevation as well as azimuth.
Thus going forward many different array types will all need to operate within the limitations defined in the 3GPP LTE standard. For example, when defining codebooks (CBs) for use in closed-loop beamforming feedback procedures in 3GPP LTE, the CBs were optimized for the ULA. In particular the four antenna CB of R8 is optimized for a uniform linear array with co-polarized elements. The 8 antenna CB of R10 is optimized for a ULA consisting of cross-polarized (XP) antennas (where each XP antenna contains two co-located antennas, one with a +45 polarization and the other with a −45 polarization). In particular the 8 antenna ULA of XP antennas can be thought of as two four-antenna ULAs, one made up of +45 polarized antennas and the other made of −45 polarized antennas and the CB design exploits this structure. Given the CB optimization toward the ULA, other array geometries will have a performance loss when using CB feedback in LTE unless there is a method to improve the performance while still being standards compliant.
In IEEE 802.16m an idea of improving the codebook performance to a UE was presented where the codebook is transformed by multiplying by a UE-specific covariance matrix which would improve the performance of non-linear arrays. The 802.16m idea is hence UE-specific and requires feedback from the UE (i.e., it is not a transparent operation). Implementing this method in an LTE system would also require a significant change to the LTE standard.
In UMB (3GPP2) a mode was present that allows the eNodeB to feed forward a codebook to the UEs. In this way the eNodeB and UEs could use a codebook that is optimized for the specific array type at the eNodeB. This method is not transparent to the UE and would require a significant change to the LTE standard to be used in LTE systems.
What is needed is a UE-transparent (and hence standards compliant) method for improving the CB feedback performance for non-ULAs is given with emphasis on the UCA.